1. Field of the Invention
The present invention relates generally to systems for aligning objects and certain embodiments relate more particularly to lithographic apparatus equipped with such an alignment system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Usually, a semiconductor device comprises microelectronic circuitry which is constructed of a plurality of patterned layers stacked on each other. Each patterned layer must have a certain alignment or overlay with the previous patterned layer(s) on which the layer is located. To obtain such a certain alignment, the lithographic apparatus comprises an alignment system which determines the positions of one or more markers on the semiconductor substrate. A marker is typically a grating. The alignment system according to the prior art uses a monochromatic light beam that is aimed at the grating. A diffraction pattern is generated by the interaction of light beam and grating, and from measurement on the diffraction orders in the diffraction pattern information is obtained relating to the position of the semiconductor substrate relative to a reference position. Then, upon exposure of the patterned layer the position of the target portion can be arranged such that a minimum alignment error occurs.
However, in practice the alignment procedure is hindered by a deformation of the alignment grating(s). An alignment grating may be deformed by the processes to create each patterned layer, especially the patterned layer directly on the grating. For example, a planarisation step of the layer directly on the grating by way of chemical mechanical polishing (CMP) may cause the lines and/or trenches of the grating to obtain an asymmetric form. The asymmetry of the lines and/or trenches influence the orders in the diffraction pattern, which may lead to an erroneous shift of the aligned position as calculated from the diffraction pattern.
From the prior art it is known that it is possible to reduce the possible error introduced by (a) deformed alignment marker(s) by using a broad-band radiation source for generating a diffraction pattern on such an alignment marker. Due to the plurality of wavelengths of the radiation source, a range of diffraction angles is generated for each diffraction order. By adapting both the wavelength range and the period of the alignment marker, it is possible to obtain a number of non-overlapping diffraction angle ranges. From measuring the signal for an angle range related to a particular order, an improved alignment can result due to out-averaging of singularities in the signal.
However, use of such a broad-band system may not be compatible for a lithographic apparatus in which the broad-band radiation must pass an optical path to reach the alignment marker. Typically, the radiation follows the optical path of the projection beam in the lithographic apparatus. In the optical path, a projection system of the lithographic apparatus is arranged to have an optimal imaging function for a specific wavelength (or a small band wavelength range). Other wavelengths may not pass the optical path as desired leading to a flawed radiation beam for diffraction. Moreover, a broad-band radiation may also cause undesired thermal effects in the optical path that affect the imaging quality of the lithographic apparatus.